MBMS (Multimedia Broadcast Multicast Service) over single frequency networks (MBSFN) has recently been specified in 3GPP for Rel-7 UMTS Terrestrial Radio Access (UTRA) systems. The MBSFN feature provides significantly higher spectral efficiency compared to Rel-6 MBMS and is primarily intended for broadcasting high bit rate demanding Mobile TV services on dedicated MBMS carriers. Since it is broadcast only, MBSFN inherently targets transmissions in unpaired frequency bands.
In SFN (Single Frequency Network) transmission, multiple base stations transmit the same waveform at the same time such that a terminal receives signals from all base stations, resulting in a behavior similar to one large cell. For UTRA systems, SFN transmission implies that a cluster of time synchronized NodeBs transmit the same content using the same channelization and scrambling codes.
SFN transmission is illustrated in FIG. 1, where a terminal, or mobile station MS, receives signals from two base stations BS1 and BS2. When using cell-specific scrambling, transmissions from the right hand side base station BS2 would represent inter-cell interference for the terminal in the adjacent cell. In a single frequency network, on the other hand, inter-cell interference shows up as additional multi-path transmission, which can be accounted for by the terminal receiver as a desired signal, resulting in considerably improved coverage.
MBSFN enhances the Rel-6 MBMS physical layers by supporting SFN operations for MBMS point-to-multipoint (ptm) transmissions on a dedicated MBMS carrier. It also supports higher service bit rates and efficient time division multiplexing of services for reducing terminal battery consumption by allowing discontinuous reception (DRX) of services. MBSFN uses the same type of channels as used for Rel-6 MBMS ptm transmissions.
In order to provide smooth integration of the MBSFN feature to any existing UTRA system, MBSFN has been specified for both FDD (Frequency Division Duplex) and TDD (Time Division Duplex) based physical layer downlink (DL) channel structures:                MBSFN based on WCDMA (Wideband Code Division Multiple Access) (FDD based)        MBSFN based on TD-SCDMA (Time Division-Synchronous Code Division Multiple Access) (TDD based)        MBSFN based on TD-CDMA (Time Division-Code Division Multiple Access) (TDD based)        
The FDD related MBSFN uses the WCDMA downlink common physical layer channels for transmission of data, and no paired uplink transmissions occur. For the TDD related MBSFN, all slots are used for downlink transmissions when networks are optimized for broadcast. Hence, no duplex occurs in MBSFN and the differences between FDD and TDD based MBSFN then mainly refer to the physical layer slot formats, the way Mobile TV services are time multiplexed and the chip rates in the case of the TDD options TD-SCDMA and 7.68 Mcps TD-CDMA. (The chip rate for the third TDD option, 3.84 Mcps TD-CDMA, is the same as used in FDD.)
When transmitting downlink in all slots, the meaning of TDD and FDD becomes blurred in the sense that no duplex occurs in broadcast. As mentioned above, the difference then basically refers to the construction of the common downlink physical channels. Therefore, in an ongoing 3GPP work item [1] the objective is to specify the WCDMA based MBSFN approach as an additional TDD option, in which all slots are dedicated for broadcast. This additional TDD option has been referred to as MBSFN Downlink Optimized Broadcast (DOB). The MBSFN DOB fulfils relevant TDD RF requirements.
In cell search, the SCH (Synchronization CHannel) is used by a terminal to determine slot and radio frame synchronization as well as to identify the group code of a cell. Given the group code of the cell, the terminal can detect the cell-specific scrambling code (and midamble code in case of TDD). The cell search procedure is normally divided into three steps:                1. Slot synchronization        2. Frame synchronization and code group identification        3. Cell-specific scrambling code detection        
The synchronization channel consists of two sub-channels, the Primary SCH and the Secondary SCH, see [2],[3]:                The Primary SCH is formed by a modulated code, the Primary Synchronization Code (PSC). This code is the same for all cells in the system. With e.g. a receive filter matched to the PSC, the terminal can locate the slot timing of the cell by detecting peaks in the matched filter output.        The Secondary SCH is formed by a repeatedly transmitted sequence of modulated codes, the Secondary Synchronization Codes (SSC), and is transmitted in parallel with the Primary SCH. The SSC indicates which of the code groups the cell-specific scrambling code belongs to and the SSC also provides the possibility to obtain frame synchronization.        
In WCDMA, and 3.84 Mcps TD-CDMA systems, the 10 ms radio frames of the synchronization channels are divided into 15 slots, each of length 2560 chips. The PSC and SSC have a length of 256 chips and the mechanism to generate these synchronization codes is the same for WCDMA and 3.84 Mcps TD-CDMA, but the allocation of the codes within the frame differs.
In the case of WCDMA, the synchronization codes are allocated in each slot as illustrated by FIG. 2, whereas in TD-CDMA there are two possible allocations of the SCH codes within the frame:                1. In slot #k, where k=0 . . . 14        2. In two slots, #k and #k+8, where k=0 . . . 6        
In WCDMA, the PSC and SSC are always allocated in the beginning of the slots, as illustrated in FIG. 2, whereas in TD-CDMA a time offset can be applied to the PSC. Furthermore, in WCDMA the Secondary SCH is formed by one sequence of SSC, whereas in TD-CDMA three SSC sequences are transmitted in parallel.
When deploying MBSFN based on WCDMA common downlink channels in UMTS unpaired frequency bands (i.e. MBSFN DOB in TDD bands), there might be some impact on roaming legacy (older) TD-CDMA terminals with respect to cell search at power-on. A legacy TD-CDMA terminal that expects to find synchronization codes in at most two slots per frame could experience longer cell search times (depending on the particular implementation), due to the deployment of the WCDMA based synchronization channel structure in the unpaired spectrum. It may have to evaluate 15 positions within a frame in order to find out that it actually cannot access the MBSFN DOB carrier.
A WCDMA (and non-MBSFN capable) terminal trying to access an MBSFN DOB carrier performs the cell search steps and then reads system information on the broadcast channel (BCH) to find out that this carrier is barred. In this case, however, the WCDMA terminal may not try to perform the cell search in unpaired frequency bands, due to pre-knowledge of their spectrum locations. On the other hand, reading a barred cell may delay the cell search at power on for a roaming WCDMA and non-MBSFN capable terminal trying to access unpaired bands.